Add case insensitive comparison for extensions on Windows

IconManager fails to work properly on Windows if supplied with path with
upper/mixed case in extension.

BUG=

Review URL: https://codereview.chromium.org/27717002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230690 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230690